I just started this because of 9.99. I'm confused if I have everything from that DLC pack I bought. The 90d counter is counting down now so I'm assuming that's the exp boost or whatever. Is the platinum the currency beside my credits. 4000 seems not alot since a new frame costs 250 or something. Am I understanding this correctly?
Yes that is correct... But you should not buy any of those frames just yet as they are earnable just by playing (if you really want to just grab 1 or 2 that really stand out to you but just know in general these are not good use of plat... Frames are overpriced).
For now, don't bother going crazy with purchases on the platinum you just bought. The key thing to buy as a new player will be inventory slots for Frames and Weapons so you can have more of them. As you play more and more you'll better understand what to do what to do with that platinum. 4000 plat can go a long way if not wasted.